Abstract: With the fixed loadings of molybdenum oxide of 13.50% and cobalt oxide of 2.11%, respectively, the Co-Mo/γ-Al₂O₃ catalysts were modified by adding different mass concentration of phosphoric acid via separate-step impregnation method. The effect of different phosphorus loadings of catalysts on the hydrodesulfurization performance was studied using middle temperature coal tar from Chifeng, Inner Mongolia. The structural properties of catalysts were characterized by the temperature programmed desorption of NH₃ (NH₃-TPD), XRD, XPS and other methods. The results show that a proper loading of phosphorus can weaken the interaction between the support and the active components, improve the dispersion of active compents on the surface of support, and promote the reduction and sulfuration of the active components and the distribution of acid, which can improve the hydrodesulfurization performance of the catalysts. The catalyst prepared with the phosphoric acid of 4% concentration exhibits the highest hydrodesulfurization activity with the sulfur removal of 96.98%. The effect of mass concentration of phosphoric acid on the hydrodesulfurization activety of the prepared catalysts is in the order of 4% >2% >6% >1% >0 >8%.